Several processes in semiconductor manufacturing involve processing wafers at low-pressure. In a typical such process, the process chamber is brought to a reduced pressure; subsequently certain process gases are introduced into the chamber to create desired process conditions.
For illustration purpose, we describe one such process in some detail. Several methods have been developed for creating thin films on wafers used in manufacturing semiconductor devices. Among the more established techniques is Chemical Vapor Deposition (CVD). Atomic Layer Deposition (ALD), a variant of CVD, is a relatively newer technology now emerging as a potentially superior method of achieving highly uniform, conformal film deposition.
ALD has demonstrated an outstanding ability to maintain ultra-uniform thin deposition layers over complex topology. This is at least partially true because ALD is not as flux dependent as is CVD. This flux-independent nature of ALD allows processing at lower temperatures than with conventional CVD methods.
The technique of ALD is based on the principle of the formation of a saturated monolayer of reactive precursor molecules by chemisorption. A typical ALD process consists of injecting a precursor RA for a period of time until a saturated monolayer is formed on the wafer. Then, the precursor RA is purged from the chamber using an inert gas, GI. This is followed by injecting precursor RB into the chamber, also for a period of time, thus forming the layer AB on the wafer. Then, the precursor RB is purged from the chamber. This process of introducing RA, purging the reactor, introducing RB, and purging the reactor can be repeated a number of times to achieve an AB film of a desired thickness.
This process is also illustrative of the types of issues encountered in such low-pressure processing. For example:
1) Film quality and composition can be greatly impacted in the case where the flow rates of the Reactants RA and RB fail to be as desired in the recipe. For example, if the MFC corresponding to the reactant fails to perform as desired to deliver the recipe setpoint flow rate value, the film quality can be affected.
2) In-situ measurements providing details of wafer condition, such as when saturation of a precursor monolayer is completed on the wafer(s), are not available; this hinders the ability to control and optimize processing conditions so as to achieve optimal performance and throughput.